One of the most abundant fossil fuels is natural gas, which is principally methane. In high temperature processes involving the conversion of methane to high value products such as liquid hydrocarbons, chemicals, such as ethylene, or the generation of electric power, environmental conditions are often encountered involving very high carbon activities and relatively low oxygen activities. Similar environments can also be encountered in many other syngas generation processes. In many syngas generation processes, for example the conversion of methane to syngas, coke to syngas, coal to syngas, heavy oils and bitumen to syngas, environments are encountered that have high carbon activities and relatively low oxygen activities. High temperature reactor materials, heat exchanger materials, and syngas process tubing and piping materials used in such processes can deteriorate in service by a very aggressive form of corrosion known as metal dusting. Metal Dusting is a deleterious form of high temperature corrosion experienced by Fe, Ni and Co-based alloys at temperatures in the range of 350-1050° C. in carbon-supersaturated (carbon activity>1) environments having relatively low (about 10−10 to about 10−20 atmospheres) oxygen partial pressures. This form of corrosion is characterized by the disintegration of bulk metal into powder or dust. Most alloys that are commercially available today degrade by this corrosion process.
Although many high temperature alloys are designed to form in-situ surface film of chromium oxide (Cr2O3) in low oxygen partial pressure environments, chromium oxide reacts at high temperatures (i.e. >1000° C.) in the presence of oxygen to form CrO3, which is a vapor and evaporates to result in a chromium-depleted alloy. The chromium depleted alloys are unable to form a protective chromium oxide film, thus carbon ingresses into the alloy from highly reducing carbon-rich environments with carbon activities in excess of unity. This results in metal dusting corrosion.
Aluminum and silicon are strong oxide formers and can be added to high temperature alloys to improve corrosion resistance by forming an in-situ surface film of aluminum oxide and silicon oxide. However, an excess addition of these elements, which is desired for superior corrosion resistance, generally leads to poor mechanical strength at elevated temperatures at which the alloys are used. Thus, alloys containing an excessive amount of aluminum and silicon can not be used in constructing components in syngas generation processes.
Methodologies disclosed in the literature for controlling metal dusting corrosion involve the use of gaseous inhibitors, for example H2S. Inhibition by H2S has two disadvantages. One is that H2S tends to poison most catalysts used in hydrocarbon conversion processes. Secondly, H2S has to be removed from the exit stream which can substantially add to process costs.
U.S. Pat. No. 6,692,838 to Ramanarayanan et al. discloses compositions resistant to metal dusting and a method for preventing metal dusting on metal surfaces exposed to carbon supersaturated environments. The compositions comprise (a) an alloy, and (b) a protective oxide coating on the alloy. The alloy includes alloying metals and base metals, wherein the alloying metals comprise a mixture of chromium manganese, and the base metal comprises iron, nickel, and cobalt. U.S. Pat. No. 6,692,838 is incorporated herein by reference in its entirety.
U.S. Pat. No. 6,737,175 to Ramanarayanan et al. discloses an alloy composition resistant to metal dusting and a method for inhibiting metal dusting corrosion of metal surfaces exposed to supersaturated carbon environments. The method includes constructing the surfaces of, or coating the surfaces with a copper based alloy. U.S. Pat. No. 6,737,175 is incorporated herein by reference in its entirety.
U.S. patent application Ser. No. 11/126,007 filed on May 10, 2005 to Chun et al. also discloses alloy compositions and methods for preventing metal dusting on metal surfaces exposed to carbon supersaturated environments. The alloy compositions include an alloy (PQR), and a multi-layer (at least three layers) oxide film on the surface of the alloy (PQR) wherein the alloy (PQR) includes a metal (P) selected from the group consisting of Fe, Ni, Co, and mixtures thereof, an alloying metal (Q) comprising Cr, Mn, and either Al, Si, or Al/Si, and an alloying element (R). The multi-layer oxide film is formed in-situ during use of the alloy composition in a carbon supersaturated metal dusting environment. U.S. patent application Ser. No. 11/126,007 is incorporated herein by reference in its entirety.
There is a need for new alloys and coating materials, which are resistant to metal dusting corrosion. More particularly, a need exists for an advanced coated material composition, wherein the coated metal is resistant to metal dusting corrosion in low (about 10−10 to about 10−20 atmospheres) oxygen partial pressure and carbon-supersaturated (carbon activity>1) environments and includes a base metal providing the coated material with the required high temperature strength and other properties, such as creep strength and toughness. Such an advanced coated material composition should be capable of forming an outer protective oxide layer to block carbon transfer by acting as a diffusion barrier to carbon ingress.